The Atari 400 and 800 were the first home computers to use custom coprocessors and the first to use "sprites" and special video interruptions like display lists, features that will be implemented later on the Commodore 64 and Amiga (Atari 400/800 and Amiga were designed by Jay Miner).
It offered high graphic resolution, lots of colors and great sound capabilities, more than other computers could do then! The two models had same characteristics, the 400 is the low-cost version, it has only 16 kb (instead of 48 kb), one cartridge port (two for the 800) and a membrane keyboard (a typewriter style keyboard one for the 800).

Originally, the 400 was sold with 8 kb RAM, but later most 400's with 16 kb.

16 (each color can have 8 luminances) = 128 colors maximum in the lowest graphic mode (requiring display list interruption to have them simultaneously) and up to 256 colors in some specific modes for machines having the GTIA chip instead of the CTIA